Alterations of ciliate phosducin phosphorylation in Blepharisma japonicum cells

We have previously reported that motile photophobic response in ciliate Blepharisma japonicum correlates with dephosphorylation of a cytosolic 28 kDa phosphoprotein (PP28) exhibiting properties similar to those of phosducin. Here we demonstrate in in vivo phosphorylation assay that the light-elicited dephosphorylation of the PP28 is significantly modified by cell incubation with substances known to modulate protein phosphatase and kinase activities. Immunoblot analyses showed that incubation of ciliates with okadaic acid and calyculin A, potent inhibitors of type 1 or 2A protein phosphatases, distinctly increased phosphorylation of PP28 in dark-adapted cells and markedly weakened dephosphorylation of the ciliate phosducin following cell illumination. An enhancement of PP28 phosphorylation was also observed in dark-adapted ciliates exposed to 8-Br-cAMP and 8-Br-cGMP, slowly hydrolysable cyclic nucleotide analogs and 3-isobutyryl-1-methylxanthine (IBMX), a non-specific cyclic nucleotide phosphodiesterase (PDEs) inhibitor. Only slight changes in light-evoked dephosphorylation levels of PP28 were observed in cells treated with the cyclic nucleotide analogs and IBMX. Incubation of ciliates with H 89 or KT 5823, highly selective inhibitor of cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), respectively, decreased PP28 phosphorylation levels in dark-adapted cells, whereas the extent of light-evoked dephosphorylation of the phosphoprotein was only slightly influenced. Cell treatment with higher Ca2+ concentration together with ionophore A23187 in culture medium resulted in marked increase in PP28 phosphorylation levels, while quite an opposite effect was observed in cells exposed to Ca2+ chelators, EGTA or BAPTA/AM as well as calmodulin antagonists, such as trifluoperazine (TFP), W-7 or calmidazolium. Light-dependent dephosphorylation was not considerably affected by these treatments. The experimental findings presented here suggest that an endogenous light-dependent protein kinase-phosphatase system may be engaged in the alteration of phosducin phosphorylation in ciliate B. japonicum thereby to modulate the cell motile photophobic behavior.     

Single-site mutation and secondary structure stability: an isodesmic reaction approach. The case of unnatural amino acid mutagenesis Ala-->Lac

A method is described to evaluate backbone interactions in proteins via computational unnatural amino acid mutagenesis. Several N-acetyl polyalanyl amides (AcA(n)NH(2)) were optimized in the representative helical (3(10)-, 4(13)-, and a "hybrid" kappa-helix, n = 7, 9, 10, 14) and hairpin (two- and three-stranded antiparallel beta-sheets with type I turns betaalphaalphaepsilon, n = 6, 9, 10) conformations, and extended conformers of N-acetyl polyalanyl methylamides (n = 2, 3) were used to derive multistranded beta-sheet fragments. Subsequently, each residue of every model structure was substituted, one at a time, with l-lactic acid. The resulting mutant structures were again optimized, and group-transfer energies DeltaE(GT) were obtained as heats of the isodesmic reactions: AcA(n)NHR + AcOMe --> AcA(x)LacA(y)NHR + AcNHMe (R = H, CH(3)). These group-transfer energies correlate with the degree of charge polarization of the substituted peptide linkages as measured by the difference Deltae in H and O Mulliken populations in HN-C=O and with the H-bond distances in the "wild-type" structures. A good correlation obtains for the HF/3-21G and B3LYP/6-31G* group-transfer energies. The destabilization effects are interpreted in terms of loss of interstrand and intrastrand H-bonds, decrease in Lewis basicity of the C=O group, and O...O repulsion. On the basis of several comparisons of Ala --> Lac DeltaE(GT)'s with heats of the NH --> CH(2) substitutions, the latter contribution is estimated (B3LYP/6-31G*) to range between 1.5 and 2.4 kcal mol(-1), a figure close to the recent experimental DeltaDeltaG(o) value of 2.6 kcal mol(-1) (McComas, C. C.; Crowley, B. M.; Boger, D. L. J. Am.Chem. Soc. 2003, 125, 9314). The partitioning yields the following maximum values of the electronic association energy of H-bonds in the examined sample of model structures (B3LYP/6-31G* estimates): 3(10)-helix D(e) = -1.7 kcal mol(-1), alpha-helix D(e) = -3.8 kcal mol(-1), beta-sheet D(e) = -6.1 kcal mol(-1). The premise of experimental evaluations of the backbone-backbone H-bonding that Ala --> Lac substitution in proteins is isosteric (e.g., Koh, J. T.; Cornish, V. W.; Schultz, P. G. Biochemistry 1997, 36, 11314) is often but not always corroborated. Examination of the integrity of H-bonding pattern and phi(i), psi(i) distribution identified several mutants with significant distortions of the "wild-type" structure resulting inter alia from the transitions between i, i + 3 and i, i + 4 H-bonding in helices, observed previously in the crystallographic studies of depsipeptides (Ohyama, T.; Oku, H.; Hiroki, A.; Maekawa, Y.; Yoshida, M.; Katakai, R. Biopolymers 2000, 54, 375; Karle, I. L.; Das, C.; Balaram, P. Biopolymers 2001, 59, 276). Thus, the isodesmic reaction approach provides a simple way to gauge how conformation of the polypeptide chain and dimensions of the H-bonding network affect the strength of backbone-backbone C=O...HN bonds. The results indicate that the stabilization provided by such interactions increases on going from 3(10)-helix to alpha-helix to beta-sheet.     

The α‐ and β‐epoxides of 3β‐acet­oxy‐5α‐lanost‐9(11)‐en‐7‐one

The structures of 3β‐acet­oxy‐9α,11α‐ep­oxy‐5α‐lanost‐9(11)‐en‐7‐one and 3β‐acet­oxy‐9β,11β‐ep­oxy‐5α‐lanost‐9(11)‐en‐7‐one, C₃₂H₅₂O₄, differ in their respective substituted cyclo­hexa­none rings but adopt similar conformations in the other three rings. In both of the crystal structures, weak inter­molecular C—H⋯O inter­actions are present.     

Reactions of Polycyclic Ketones with Dimethoxycarbene; a Convenient Route for a ‘One‐Pot’ Preparation of Some α‐Hydroxycarboxylic Acid Esters

Polycyclic ‘cage’ ketones, such as pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecan‐8‐one ( 10 ), pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane‐8,11‐dione ( 11 ), and adamantan‐2‐one ( 16 ) were treated with the nucleophilic dimethoxycarbene (DMC; 1 ), which was generated thermally from 2,5‐dihydro‐2,2‐dimethoxy‐5,5‐dimethyl‐1,3,4‐oxadiazole ( 4a ) in boiling toluene. In this ‘one‐pot’ procedure, the α‐hydroxycarboxylic acid ester 12 or a corresponding derivative 15 or 17 was obtained (Schemes 4–7). Additionally, ‘cage’ thione 21 was treated with DMC under the same conditions yielding dimethoxythiirane 22 (Scheme 8). Subsequent hydrolysis or desulfurization (followed by hydrolysis on silica gel) of 22 gave α‐mercaptocarboxylate 25 and the corresponding desulfurized ester 24 , respectively. In all cases, the addition of DMC occurred stereoselectively, and the addition from the exo‐face is postulated to explain the structures of the isolated products.     

Energetics of proton transfer between carbon atoms (H3CH ? CH3)−

Ab initio calculations were carried out to study the potential energy surface of (H₃C? H? CH₃)^?. The 6–31G* basis set is supplemented by a set of diffuse p functions on both C and H (with a range of exponents for the latter). The binding energy of CH₄ and CH₃^? to form the (H₃CH? CH₃)^? complex is about 2 kcal/mol, much smaller than for comparable ionic H-bonded systems involving O or N atoms. Nearly half of this interaction energy is due to correlation effects, computed at second and third orders of Møller-Plesset perturbation theory. Correlation is also responsible for substantial reductions in the energy barrier to proton transfer within the complex. This barrier is computed to be 13?15 kcal/mol at the MP3 level, depending upon the exponent used for the H p functions.     

Synthesis of multihydroxyl branched polyethers by cationic copolymerization of 3,3‐bis(hydroxymethyl)oxetane and 3‐ethyl‐3‐(hydroxymethyl)oxetane

The cationic ring‐opening polymerization of 3,3‐bis(hydroxymethyl)oxetane (BHMO) and the copolymerization of BHMO with 3‐ethyl‐3‐(hydroxymethyl)oxetane (EOX) were studied. Medium molecular weight polymers (number‐average molecular weight ≈ 2 × 10³) were obtained in bulk polymerization. Poly[3,3‐bis(hydroxymethyl)oxetane], as highly insoluble, was only characterized by gel permeation chromatography and NMR methods in the esterified form. Copolymers of BHMO and EOX that were slightly soluble in organic solvents were characterized in more detail. In a copolymerization from a 1:1 mixture, the comonomers were consumed at equal rates. Matrix‐assisted laser desorption/ionization time‐of‐flight analysis confirmed that a random 1:1 copolymer was formed. ¹³C NMR analysis indicated that in contrast to previously described homopolymers of EOX in which the degree of branching was limited, the homopolymers of BHMO were highly branched. This pattern was preserved in the copolymers; EOX units were predominantly linear, whereas BHMO units were predominantly branched. The copolymerization of BHMO with EOX provides, therefore, a route to multihydroxyl branched‐polyethers with various degrees of branching containing ? OH groups exclusively as ≡C? CH₂? OH units. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1991–2002, 2002     

Kinetics and mechanism of the cationic polymerization of tetrahydrofuran in solution. I. THF–CCI4 system

Polymerization of THF in CCl₄ solvent was initiated with 1,3-dioxolan-2-ylium eations with AsF₆^?, PF₆^?, and SbF₆^? anions as well as with esters of fluorosulfonic and trifluoromethanesulfonic acids. With these esters polymerization proceeds with a marked acceleration period, due to slow initiation. The corresponding rate constants of initiation and their dependence on the polarity of the THF/CCl₄ mixture were determined. The rate constant of propagation on the macroion-pairs (k_p^±) of the polytetrahydrofurylium cation with AsF₆^?, PF₆^?, and SbF₆^? and CF₃SO₃^?, anions was found to be independent in CCl₄ solvent on the anion structure and given by the expression: k_p^± = 2.93 × 10^?2 exp {?4.7 × 10³/T} at [THF]₀ = 8.0M. This constant depends on the polarity of the polymerization mixture, and at 25°C for the THF-CCl₄ system, k_p^± = 1.78 × 10^?2 exp {?4.9/D}; thus, in CCl₄ at [THF]₀ = 8.0M, and at 25° k_p^± = 4.0 × 10^?21/mole-sec. In the polymerization with derivatives of CF₃SO₃H (able to form the corresponding macroester) the overall polymerization rate is much lower than that with complex anions because of the reversible conversion of the macroion-pairs into the macroester (internal return). The macroester is much less reactive than the macroionpair (10²–10³ times) in the monomer addition reaction. At [THF]₀ = 8.0M and at 25°C, 96.5% of the growing species exists in the macroester form. Polymerization of THF initiated with derivatives of CF₃SO₃H is a subject of a strong special salt-effect. At a sufficiently high ratio of [AgSbF₆] to [I]₀, where the initiator I is C₂H₅OSO₂CF₃, the overall polymerization rate is equal to that observed for the polymerization of THF on the macroion-pairs, since the internal return within the triflate ion-pair (the macroester formation) is eliminated and polymerization proceeds on the macroion-pairs with SbF₆- anions exclusively.     

α-phosphoryl sulfoxides. X. A general synthesis of optically active α-chlorovinyl sulfoxides

A general and efficient synthesis of enantiomeric α-chlorovinyl p-tolyl sulfoxides 1 using (+)-(S)_C(S)_S-α-chloro-α-dimethoxyphosphorylmethyl p-tolyl sulfoxide as a key substrate for the Horner-Wittig reaction with carbonyl compounds is described. The ratio of geometrical isomers of 1 was determined and briefly investigated. The X-ray diffraction structures of (Z)-1-chloro-1-p-tolylsulfinyl-2-phenyl-ethene and (Z)-1-chloro-1-p-tolylsulfinyl-2-(2-thienyl)-ethene are reported.     

Study of the mechanism of base induced dehydrobromination of trans-β-bromostyrene

Observation that rates of dehydrobromination of trans-β-bromostyrene (1) and the Hofmann degradation of tetrabutyl ammonium cation depend on strength of base in different ways and that treatment of 1 with base results in fast abstraction of the β-proton imply the possibility that the dehydrobromination of 1 could proceed via α-elimination and Ph migration. In order to clarify this question, β-¹³C-labeled 1 was obtained and subjected to PTC dehydrobromination which proceeds without migration of Ph. The obtained results are consistent with an irreversible E1cB mechanism.     

Elucidation of the vicarious nucleophilic substitution of hydrogen mechanism via studies of competition between substitution of hydrogen, deuterium, and fluorine.

Relations of rates of the vicarious nucleophilic substitution of hydrogen (VNS) and S(N)Ar substitution of fluorine in 2-fluoronitrobenzenes with chloroalkyl aryl sulfone carbanions were determined from competitive experiments carried out at various concentrations of base. The observed dependence of the VNS/S(N)Ar rate ratio on the base concentration confirmed the two-step mechanism of the VNS, which consists of reversible formation of sigma(H) adducts of the alpha-chlorocarbanion to nitroarene, followed by base-induced beta-elimination of HCl. It was also evidenced that both of these processes can be the rate-limiting steps: the beta-elimination at low base concentration and the nucleophilic addition at high base concentration. Consistent with that conclusion is the finding that the kinetic isotope effect in the VNS reaction decreases from 4.2 (a value typical of a primary KIE) to 0.8 (a value typical of a secondary KIE) with increasing base concentration. Also reported is our discovery that the S(N)Ar substitution of the 2-fluoronitrobenzenes studied in this work was subject to base catalysis under some of the experimental conditions employed in our competitive experiments.